Does anyone know how long an open bottle of Panacur lasts?

I'm not sure if it goes "off"

I have used a part bottle that had been opened for almost 12 months, and it was fine.

It has a very long shelf life (several years) as long as it is stored properly.

I have recently spoken to my vet about this, having just had my 1st bottle ;-) ....Room temp. cool( obviously not the kitchen as mine is very small and warm!) and it will do her for 6 doses so 6 months for me :-)

A preloaded syringe lasts me 12 months. I use it for my tortoises.

> I have used a part bottle that had been opened for almost 12 months, and it was fine.

I'm sure it was but I am a little curious how you could tell :) or do you mean it was within date and correctly stored?
If incorrectly stored, for instance, you would not be able to tell visually if the active components had been destroyed.  Some ingrediants may even degrade into something harmful.
As a by the by, it is also worth mentioning that if a bottle is out of date or you think it has not be correctly stored it should not be disposed off in domestic waste that may go to landfill.  Either take it back to the retailer or contact your local council about disposal.

I think it says to keep it in the fridge once you've opened it...  (Or perhaps that's just my interpretation of 'cool'.)

If refrigeration's needed it usually says so specifically; "Store in a cool place" is different. :-)